Sending messages to authors of third party clients that you are not ok with their use of your servers (literally the exact comment the link I and other posters shared):
> I'm not OK with LibreSignal using our servers, and I'm not OK with LibreSignal using the name "Signal." You're free to use our source code for whatever you would like under the terms of the license, but you're not entitled to use our name or the service that we run.
> If you think running servers is difficult and expensive (you're right), ask yourself why you feel entitled for us to run them for your product.
Yes, they mention both trademarks _and_ servers, and yes, if there was not a trademark issue, github and google would not remove the repo just for connecting to Signal's servers against Moxie's wishes.
However, the act of informing third party client developers that they are not allowed use the official servers is itself an act of enforcement - maybe one with not much teeth behind it unless he follows up with a legal complaint, but still nonetheless enforcement.